As lubricants for magnetic disks, perfluoropolyether compounds are generally used. Among various perfluoropolyether compounds, Fomblin-based (product of Solvay Solexis K.K.) compounds having low surface energies and low frictional forces are most frequently used. Fomblin-based compounds are perfluoropolyether compounds wherein the basic skeleton of its main chain is (CF2CF2O)m(CF2O)n.
However, it is known that Fomblin-based compounds react with alumina (Al2O3) contained in head members, so that cleavage of the main chain can occur (Macromolecules, 1992, Vol. 25, pp. 6791-6799). As this cleavage progresses, the perfluoropolyether is depolymerized and eventually volatilized from the magnetic disk.
There are two modes for driving a magnetic disk drive, namely contact start/stop (CSS) wherein the head and the disk come into contact when the device starts or stops, and ramp loading/unloading (L/UL) wherein the head retracts to outside the disk surface when the device stops. The properties required of a lubricant vary depending on the mode employed.
In recent years, there has been a demand for increased storage densities and processing speeds of disk units. In order to meet such demand, it is necessary to decrease the distance between the head and disk (flying height) and/or accelerate the rotation of the disk. Even in the L/UL mode, wherein the head basically does not come into contact with the disk, with a decrease in flying height and increase in rotational speed, the head comes into contact with the disk more often, and this results in transfer of the lubricant from the surface of the disk to the head and/or splashing of the lubricant beyond the disk. This may damage the disk, and therefore a lubricant having a strong adhesion to the surface of the disk is preferable. An example of such a lubricant is a Fomblin-based perfluoropolyether compound having a plurality of polar groups (e.g., “Z-Tetraol”, product of Solvay Solexis K.K.); however, this lubricant cannot solve the problems with decomposition due to alumina.
Compounds in which an end of the perfluoropolyether skeleton is modified with a phosphazene functional group (specifications of U.S. Pat. Nos. 6,608,009 and 6,605,335) are known as examples of compounds that can reduce decomposition due to alumina despite containing a perfluoropolyether chain. These compounds prevent decomposition due to alumina by the effect of the modifying phosphazene functional group. These compounds have a high compatibility with other perfluoropolyether compounds and have an excellent CSS resistance. However, they have a low adhesion to the disk surface and therefore are not particularly preferable as lubricants for use in the L/UL mode.
In order to increase the speed of recording/reading of information, it is expected that rotational speeds of magnetic disks will reach 15,000 rpm or more in the near future. As the rotational speed of a magnetic disk increases, the amount of lubricant splashed tends to be increased. If sliding is conducted for a long time while rotating the disk at a high speed, the lubricating layer becomes thin due to decomposition, and the magnetic disk eventually breaks. In order to surely obtain reliability of magnetic disk drives, use of a lubricant having a strong adhesion to the magnetic disk and which dose not readily decompose is necessary.